Aerofoils

ABSTRACT

1,151,431. Boundary layer control. NATIONAL RESEARCH DEVELOPMENT CORP. 8 June, 1967 [10 June, 1966], No. 25967/66. Heading F2R. An aircraft wing 1 with an aspect ratio of at least 2:1 and a sweep back of at least 15 degrees, comprises a passage 12 to draw air from the boundary layer, particularly on the suction surface, and eject it from an opening near the aerofoil tip, the passage being formed between the wing and a skin 11 spaced from the wing, and providing an outer surface of aerofoil form. The skin extends for preferably about 25% of the span of the wing 1 from its tip. The skin 11 may form a complete sleeve, Fig. 4, may cover only the top surface and nose of the wing 1, Fig. 6 (not shown), and may cover only the top surface of the wing, being attached at its leading edge to the wing, Fig. 11 (not shown). The air drawn off from the boundary layer may be ejected either from the wing tip, Fig. 7, or from the trailing edge, Fig. 10. The wing may be narrowed near its tip, as at 13, Fig. 7, so that the outer surface of the skin 11 appears as a continuation of the remainder of the wing.

A. D. YOUNG AEROFOILS 3 SheetsSheet 1 Filed June 9, 1967 A. D. YOUNG Oct. 22, 1968 AEROFOILS s Shets-Sheet 2 Filed June 9, 1967 Oct. 22,1968 A. D. YOUNG 3,406,929

AEROFOILS Filed June 9, 1967 5 Sheets-Sheet 5 United States Patent 3,406,929 AEROFOILS Alec David Young, Buckhurst Hill, England, assignor to National Research Development Corporation, London, England, a corporation of Great Britain Filed June 9, 1967, Ser. No. 644,869 Claims priority, application Great Britain, June 10, 1966, 25,967/ 66 8 Claims. (Cl. 244-40) ABSTRACT OF THE DISCLOSURE The outboard section of a swept-back aerofoil has a superimposed sleeve spaced from the surface to provide an escape path under low speed-high incidence conditions, for outwardly drifting tired boundary layer air which is drawn out at the aerofoil-tip region by the tip trailing vortex, the exterior of the sleeve generating a new boundary layer.

The sleeve may be discontinued over the whole or over the trailing edge region of the underside of the aerofoil.

This invention relates to aerofoils.

According ot the invention there is provided an aerofoil of not less than approximately 2:1 aspect ratio and with a sweep-back of not less than approximately 15 to a line normal to the direction of motion of the aerofoil characterised by a sleeve spaced away from the main aerofoil surface and covering at least the upper surface thereof in the region of the tip of the aerofoil, the said sleeve. presenting, at the edge of the sleeve remote from the tip of the aerofoil, an entry opening to air moving spanwise of the aerofoil such opening extending forwardly from the trailing edge of the aerofoil for a substantial part of the chord and permitting passage of air into the space between the sleeve and the aerofoil surface within the sleeve, and an opening located in the region of the aerofoil tip, for exit of the said air from the said space, the outer surface of the said sleeve having an aerofoil shape capable of producing lift.

The invention will be more readily understood from the following description of certain embodiments thereof illustrated in the accompanying drawings in which:

FIGURE 1 shows the pattern of air flow over the upper surface of a normal swept-back aircraft wing at an incidence appropriate to low speed flight,

FIGURE 2 shows the pattern of air flow over the wing of FIGURE 1, in cross section in a plane indicated by the line II-II in FIGURE 1,

FIGURE 3 shows the pattern of air flow over the wing of FIGURE 1, in cross section in a plane indicated by the line IIIIII in FIGURE 1,

FIGURE 4 shows a wing similar to that of FIGURE 1 equipped with a sleeve according to the invention,

FIGURE 5 is a cross section in a plane indicated by the line V-V in FIGURE 4,

FIGURE 6 is a cross section of a modification of the embodiment of FIGURE 4,

FIGURE 7 shows a modified form of the invention, in longitudinal section in a vertical span-wise plane,

FIGURE 8 shows a further modified form of the invention, being a plan of the wing tip region,

FIGURE 9 is a span-wise vertical section of part of the 'ice wing of FIGURE 8, in a plane indicated by the line IXIX in FIGURE 8,

FIGURE 10 is a cross section of the wing of FIGURE 8 in a plane indicated by the line X-X in FIGURE 8,

FIGURE 11 is a cross section of a wing according to yet another embodiment of the invention, and

FIGURE 12 is a perspective sketch illustrating a method of attaching the sleeve to the Wing.

The invention is particularly, but not exclusively, suitable for use in transport aircraft required to cruise at high subsonic speeds the wings of which must have a moderate to high sweep-back to secure the required performance.

At the high angles of incidence involved in landing and take-off, however, it is difficult to achieve an adequate maximum lift co-efiicient and there is a tendency for a pronounced nose-up change of trim to occur at low speeds, accompanied by a reduction of longitudinal stability. Sometimes also it is diflicult to achieve adequate lateral control.

These difficulties stem from a large-scale flow separation which tends to develop near the wing tips at angles of incidence approaching those of landing and take-off, which results in loss of lift in such regions.

The flow pattern over swept wings at the incidences appropriate to low speed flight is complex and difficult to describe in simple terms. However, what is generally observed is an early separation extending outboard from somewhere along the span; in any otherwise vertical plane this is usually followed by re-attachment but the region .of separated flow grows with distance outboard along the span and is likely to be extensive over the tips, The region of separation takes the form of a bubble which increases in extent from its inboard end outwards and in the rearmost quarter of the bubble there is embedded a vortex (sometimes called a part span vortex) the axis of which is swept across the wing at an angle greater than the geometric sweep angle of the wing. Inside the bubble but ahead of the vortex there is a dead air region in which the pressure is nearly constant of such a value that little lift is generated there. With increase of incidence the inboard end of the separation bubble and its vortex move inwards and the extent of the separation region over the tip area grows until in that area the separation extends over the whole chord and there is no re-attachment. In addition there is a spanwise movement of air both within the rear part of the separation bubble (particularly in the vortex region) and downstream of it "(see FIG- URE 1).

As long as the chordwise extent of the separation bubble is small compared with the local chord its effect on the local lift loading is small, but where the chordwise extent is a significant fraction of the local chord there is a marked reduction of lift.

This is illustrated in FIGURES 1, 2 and 3 where a wing 1 is moving forward in the direction of arrow 2. Lines 3, 4 and 5 are typical stream lines which show a progressively increasing spanwise component of direction from wing root to wing tip.

In the region indicated by line IIII as shown in FIG- URE 2 separation takes plane near the leading edge 6 giving rise to separation bubble 7 but re-attachment takes place at 8 and substantial lift is retained. In the region indicated by line III-III however, as shown in FIGURE 3, the separation bubble has extended to the trailing edge 3 and separation of the air stream occurs over substantially the whole chord and there is a drastic fall in the lift which is even more marked further outboard.

The part-span vortex referred to above, is indicated by the dotted line envelope 9, in FIGURE 1 and by ovals 9 in FIGURES 2 and 3. Outboard of vortex 9 there is a dead air region 10, coarsely hatched in the drawing.

Thus, we find that the tip regions of swept wings show an early tendency to become inefficient as areas for generating lift and as incidence increases more and more of the lift on the wing tends to be provided by the inboard parts of the wing. This is the reason for the nose-up change of trim and the loss of longitudinal stability and reduction of lateral control with increase of incidence.

Fences and vortex generators are devices currently in use to overcome these difficulties. Their application is essentially ad hoc; they generally involve performance penalties and they are not always as effective as would be desired.

The invention meets these difiiculties by providing an escape route for the outwardly drifting tired boundary layer air along the span towards the wing tip, between the main wing surface and a super-imposed wing surface spaced away from the main wing surface. The boundary layer is referred to as tired because it has lost much of its kinetic energy in traversing the wing surface and the thicker the boundary layer the more easily does it separate from the wing surface. However, over the sleeve a new and more vigorous boundary layer forms.

One form of the invention is illustrated in FIGURES 4 and 5 in which a wing 1, similar to that shown in FIG- URE 1, is enveloped at the tip region by a sleeve 11 spaced away from the surface of the wing so as to leave a gap 12.

The outwardly drifting boundary layer, as indicated in FIGURE 1 flows along the gap 12 towards the wing tip and is sucked out of gap 12 by the tip trailing vortex, the outer surface of sleeve 11 generating a new boundary layer without the separation described in relation to FIG- URE 1 and generating a much greater lift than would be the case in the absence of sleeve 11.

Sleeve 11 is supported from wing 1 in any convenient manner, for instance by a number of posts bridging gap 12, but disposed so as to provide minimum obstruction of the air flow through gap 12. An example of this method of supporting sleeve 11 is described below in relation to FIGURE 12.

As the air flow disturbances referred to in relation to FIGURE 1 occur principally on the upper surface of the wing a modified form of the invention has most of the lower part of sleeve 11 omitted, only sufficient remaining to provide a well-shaped leading edge for the outer surface of the sleeve which remains. This variant is shown in FIGURE 6.

Where it is desired to reduce to a minimum the high speed performance penalty due to the use of the sleeve 11, the wing may be reduced in thickness, and possibly also in chord so that th outer contour of sleeve 11 may be a natural continuation of the wing surface of the wing 1 inboard of the sleeve 11. This is shown in FIGURE 7. Inevitably there is a gap 13 in the otherwise continuous surface of the wing and sleeve to provide an entry for the outwardly drifting boundary layer into gap 12.

A further embodiment of the invention is illustrated in FIGURES 8, 9, and 10. The sleeve 11 is continuous around the wing tip but an opening 14 is left at the trailing edge, at least in the vicinity of the wing tip, where the suction of the trailing vortex system may be effective to induce a flow rough gap 12 between the sleeve 11 and the wing surface.

It is not necessary that the depth of gap 12 should be uniform across the chord of the wing and it may be smaller at the leading edge as illustrated in FIGURE 11 which shows a sleeve similar to that of FIGURE 6 except that it is not carried round the leading edge of the wing but is attached thereto along the upper surface of the leading edge so as to provide a gap which increases in depth chordwise from leading edge to trailing edge.

FIGURE 12 illustrates a method of fixing sleeve 11 to the wing. The wing structure comprises spanwise main spans 16 of girder section and chordwise ribs 17, covered by a skin 18 secured for instance by rivets 19. Hollow posts 20 and 21 protrude from the skin 18 at the junction of the spans 16 and the ribs 17 and the sleeve 11 is fixed to these in any convenient manner. The posts could be shrouded by or take the form of streamlined fairings, each aligned with the local direction of flow of air through gap 12, which could be determined by wind tunnel tests.

In the drawing skin 18 and sleeve 11 are cut away to reveal the spans 16, ribs 17 and sleeve supports 20, 21 more clearly.

If required the sleeve could be combined with the provision of leading edge droop and it need not interfere with the operation of ailerons and trailing edge flaps. For this the hinged leading edge of the wing would preferably have a separate section of the sleeve attached to it which would accommodate the drooping movements of the hinged leading edge by overlapping the adjacent edges of the remainder of the sleeve and sliding thereover.

The spanwise extent of the sleeve 11 is not critical but a width of the order of A of the total wingspan is preferred. It should be as small as possible but /6 of the total wing span is about the minimum for effectiveness.

The optimum depth for the gap 12 depends on a number of factors connected with the wing section and other design parameters; in general a depth of the order of 2% or 3% of the chord should be adequate. It should be as small as possible however to reduce the bulk of the sleeve.

I claim:

1. An aerofoil of not less than approximately 2:1 aspect ratio and with a sweep-back of not less than approximately 15 to a line normal to the direction of motion of the aerofoil characterised by a sleeve spaced away from the main areofoil surface and covering at least the upper surface thereof in the region of the tip of the aerofoil, the said sleeve presenting at the edge of the sleeve remote from the tip of the aerofoil, an entry opening for air moving spanwise of the aerofoil such opening extending forwardly from the trailing edge of the aerofoil for a substantial part of the chord and permitting the passage of air into the space between the sleeve and the aerofoil surface within the sleeve, and an opening located in the region of the aerofoil tip, for exit of the said air from the said space, the outer surface of the said sleeve having an aerofoil shape capable of producing lift.

2. An aerofoil as claimed in claim 1 in which the spanwise extent of the sleeve is between /6 and A of the span of the aerofoil.

3. An aerofoil as claimed in claim 1 in which the sleeve is spaced away from the areofoil surface by a distance between 2% and 3% of the chord of the aerofoil.

4. An aerofoil as claimed in claim 1 in which the sleeve is discontinued over at least a proportion of the chordwise extent of the underside of the aerofoil including the trailing edge region.

5. An aerofoil as claimed in claim 1 in which the sleeve extends over the upper part of the aerofoil only, bging attached to the aerofoil surface, along its leading e ge.

6. An aerofoil as claimed in claim 1 in which the said exit opening is aligned with the trailing edge of the aerofoil, the sleeve being extended around and spaced from the tip of the aerofoil.

7. An aerofoil as claimed in claim 1 in which the sleeve is continued around and spaced from both the leading and trailing edges of the aerofoil, the exit open- 5 6 ing being aligned with the aerofoil tip and extending at References Cited least over rearward region thereof. UNITED STATES PATEN 8. An aerofoil as claimed in claim 1 in which the part T of the aerofoil surface over which the sleeve extends is 2'661167 12/1953 Clark 24440 X recessed in relation to the remainder of the aerofoil sur- 5 2833492 5/1958 Fowler 244-40 face and 1n whlch the outer surface of the sleeve con- MILTON BUCHLER Primary Examiner tlnues the surface of the said remainder, save for a gap forming the said entry opening, T. MAJOR, Assistant Examiner. 

